UC Berkeley

Epithelia provide the fundamental building block for nearly all Eumetazoan organs. These tissues provide a barrier between the organism and its surroundings, facilitating absorption and secretion of necessary macromolecules, while protecting the animal from harm. To perform these functions, epithelia polarize along their apicobasal axis, forming an apical domain that faces the environment or lumen, and a basal domain that attaches to the extracellular matrix. Maintenance of apicobasal polarity is critical for normal organ function, and defective architecture has been linked to disease. Most importantly, loss of tissue polarity is associated with increased malignancy of human tumors. However, the mechanisms coupling epithelial polarity to the suppression of tumor formation are unknown.

The identification of regulators of epithelial architecture revealed that the core apical and basolateral determinants are evolutionarily conserved. Thus, the epithelial tissues of Drosophila provide a genetically tractable model for studying the relationship between polarity and tumor suppression. In Drosophila epithelia, disruption of apical membrane regulators leads to cell death, while mutation of basolateral determinants gives rise to the strikingly opposite phenotype: the formation of massively overproliferating apolar tumors that fail to differentiate and exhibit increased metastatic potential. Due to these cancer-like phenotypes, genes encoding the basolateral regulators have been termed neoplastic tumor suppressor genes (nTSGs). Although these data demonstrate an intimate link between polarity and proliferation control, it remains mysterious how disorganization at the plasma membrane leads to the downstream transcriptional changes required for neoplastic tumor formation.

Studies of nTSG mutant tissue have identified several signaling pathways activated upon polarity loss. In Chapter 1, I review the recent findings implicating these pathways and their known downstream transcriptional targets to tumor formation. Specifically, I highlight the roles of the stress-activated Jun kinase (JNK) cascade, the apical regulator, atypical Protein Kinase C (aPKC), and the tumor suppressive Hippo (Hpo) pathway in neoplastic overgrowth. In addition, I discuss the controversies surrounding how these pathways interact to modulate downstream transcription.

A key unanswered question that follows from these findings is: how is polarity status at the plasma membrane transduced into expression alterations of oncogenic target genes in the nucleus? To address this outstanding issue, in Chapter 2, I analyze the gene expression profile of neoplastic tumors and identify transcriptional signatures of neoplasia as well as specific functional targets mediating overproliferation. To assess the interactions of downstream pathways driving transcription, I isolate a polarity-responsive enhancer element at a functionally important locus and evaluate its response to aPKC, JNK and Hpo signaling. My results demonstrate that, although JNK is necessary for neoplastic tumor formation, aPKC is a key mediator of gene expression alterations, via the Hpo pathway transcription factor Yorkie. These findings provide a model for how polarity-sensitive signaling pathways synergize to activate mitogenic target genes, leading to tumor formation.

In Chapter 3, I demonstrate additional regulation of polarity-responsive enhancer elements by the chromatin-modifying Polycomb Group genes, a recently identified TSG class. Further, I use molecular, genetic, and genomic approaches to evaluate the inter-relationship between the polarity-regulating and chromatin-modifying TSGs in modulating downstream target expression. Based upon my findings, I propose a model where misregulation of chromatin-modifying TSGs upon polarity loss potentiates target enhancers for activation by the polarity-responsive signaling pathways.

Taken together, my thesis work reveals that polarity loss triggers architecture disruption, the activation of stress signaling, dedifferentiation, and mitogenic gene expression. Interestingly, these responses, including PcG-mediated transcriptional derepression, are similarly elicited upon epithelial damage, such as wounding. After a wound response, however, the epithelium is repaired, and these signals are abrogated to maintain homeostasis. In contrast, epithelial integrity is never restored in polarity-deficient tissues and these signals persist, leading to the formation of a neoplastic tumor. These data suggest that Drosophila tumors behave like `wounds that never heal', paralleling previously described models of human tumors.